Systems and Methods for Achieving Partial Nitrification in a Biological Nitrogen Removal Reactor

ABSTRACT

Methods of controlling a nitrification reaction in a biological nitrogen removal reactor to favor partial nitrification of ammonia to nitrite instead of complete oxidation of ammonia to nitrate are disclosed. In some embodiments, the methods include the following: maintaining a pH in the reactor within a range that promotes growth of ammonia oxidizing bacteria; maintaining a concentration of dissolved oxygen in the reactor within a range that limits the ammonia oxidizing bacteria from completing nitrification; selecting an operational solids retention time within a range suitable for maintaining increasing concentrations of the ammonia oxidizing bacteria in the reactor while reducing concentrations of nitrite oxidizing bacteria in the reactor; and increasing a concentration of free ammonia in the reactor thereby inhibiting growth of the nitrite oxidizing bacteria in the reactor.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.12/681,123, filed Apr. 1, 2010, which is incorporated by reference as ifdisclosed herein in its entirety.

BACKGROUND

Nitrogen containing compounds, primarily ammonia, are a serious waterpollutant, which governments have begun regulating more strictly.Combined with increasing population pressures, the need for an efficientmethod for removing nitrogen from sewage is growing. Nature's ownnitrogen cycle employs specialized bacteria to convert ammonia tonitrites and nitrates. Different bacteria then convert these productsinto inert, atmospheric nitrogen gas. Waste treatment plants employthese same bacteria to perform “biological nitrogen removal” (BNR).

Conventional BNR is achieved by complete oxidation of ammonia to nitrate(nitrification) followed by the reduction of nitrate to dinitrogen gas(denitrification). The typical removal of ammonia involves its oxidationby nitrifying bacteria into nitrite (NO₂ ⁻), which is then furtheroxidized into nitrate (NO₃ ²⁻). The result is a mixture of nitrite andnitrate. Denitrifying bacteria then covert both nitrite and nitrate intonitrogen gas (N₂). The overall result is the conversion of ammonia, aharmful water pollutant, into harmless nitrogen gas, the major componentof Earth's atmosphere. Each step of nitrification/oxidation anddenitrification/reduction requires resources, in the form of energy,aerated oxygen, and an electron source such as methanol.

The annual costs of treating U.S. wastewater alone are estimated to be$25 billion and escalating. It is also estimated that many more billionswill be needed in future decades to maintain and replace ageinginfrastructure. Furthermore, expanding wastewater infrastructure toaccommodate an increasing population adds to this cost. Globally, thereis an urgent need for lower-cost water treatment technologies indeveloping countries and rural areas.

If engineering based control of nitrification could be achieved toresult in partial oxidation of ammonia solely to nitrite, it isestimated that a 25% savings in aeration cost could be realized.Correspondingly, denitrification based on nitrite rather than nitratecould result in up to a 40% savings on electron donor costs.

SUMMARY

Methods of controlling a nitrification reaction in a biological nitrogenremoval reactor to favor partial nitrification of ammonia to nitriteinstead of complete oxidation of ammonia to nitrate are disclosed. Insome embodiments, the methods include the following: maintaining a pH inthe reactor within a range that promotes growth of ammonia oxidizingbacteria; maintaining a concentration of dissolved oxygen in the reactorwithin a range that limits the ammonia oxidizing bacteria fromcompleting nitrification; selecting an operational solids retention timewithin a range suitable for maintaining increasing concentrations of theammonia oxidizing bacteria in the reactor while reducing concentrationsof nitrite oxidizing bacteria in the reactor; and increasing aconcentration of free ammonia in the reactor thereby inhibiting growthof the nitrite oxidizing bacteria in the reactor.

Systems for achieving partial nitrification in a biological nitrogenremoval reactor are disclosed. In some embodiments, the systems includethe following: a measurement module including testing apparatus formeasuring pH, dissolved oxygen, and operational solids retention time inthe reactor; a pH control module for comparing pH measured in thereactor to a predetermined range for promoting growth of ammoniaoxidizing bacteria and adjusting pH in the reactor if it is not withinthe predetermined range so that it is within the predetermined range; adissolved oxygen control module for comparing dissolved oxygen measuredin the reactor to a predetermined range for limiting the ammoniaoxidizing bacteria from completing nitrification and adjusting dissolvedoxygen in the reactor if it is not within the predetermined range sothat it is within the predetermined range; and an operational solidsretention time control module for comparing the operational solidsretention time of the reactor to a predetermined range suitable formaintaining increasing concentrations of the ammonia oxidizing bacteriain the reactor while reducing concentrations of nitrite oxidizingbacteria in the reactor and adjusting operational solids retention timein the reactor if it is not within the predetermined range so that it iswithin the predetermined range.

Methods of controlling a nitrification reaction in a biological nitrogenremoval reactor to favor partial nitrification of ammonia to nitriteinstead of complete oxidation of ammonia to nitrate are disclosed. Insome embodiments, the methods include the following: maintaining a pH inthe reactor about 7.4 to 7.6 so as to promote growth of ammoniaoxidizing bacteria; maintaining a concentration of dissolved oxygen inthe reactor within a range that limits the ammonia oxidizing bacteriafrom completing nitrification; selecting an operational solids retentiontime within a range suitable for maintaining increasing concentrationsof the ammonia oxidizing bacteria in the reactor while reducingconcentrations of nitrite oxidizing bacteria in the reactor; andincreasing a concentration of free ammonia in the reactor therebyinhibiting growth of the nitrite oxidizing bacteria in the reactor.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings show embodiments of the disclosed subject matter for thepurpose of illustrating the invention. However, it should be understoodthat the present application is not limited to the precise arrangementsand instrumentalities shown in the drawings, wherein:

FIG. 1 is a schematic diagram of systems and methods according to someembodiments of the disclosed subject matter;

FIG. 2 is a schematic diagram of systems according to some embodimentsof the disclosed subject matter;

FIG. 3 is a diagram of a method according to some embodiments of thedisclosed subject matter;

FIG. 4 is a chart of ammonia removal and nitrite accumulation duringoperation of systems and methods according to the disclosed subjectmatter; and

FIG. 5 is a chart of AOB and NOB biomass COD concentrations (X_(AOB) andX_(NOB)) measured in real time using qPCR testing for systems andmethods according to the disclosed subject matter.

DETAILED DESCRIPTION

Referring now to FIG. 1 and as mentioned above, conventional BNRincludes nitrification followed by denitrification. Nitrificationincludes first using AOB to oxidize ammonia to nitrite (NO₂ ⁻) (AOB StepI) and then using NOB to further oxidize the nitrite to nitrate (NO₃ ²⁻)(NOB Step II). Denitrification includes the reduction of nitrate, andany remaining nitrite, to nitrogen gas (N₂) (DNB Steps I . . . n). Asshown in FIG. 1 and indicated by the center vertical arrow extendingfrom the nitrite box at twelve o'clock to the nitrogen gas box at sixo'clock, partial nitrification avoids the production of nitrates andcauses direct denitrification from nitrite to nitrogen gas therebyincreasing the efficiency of the process.

Partial nitrification results from selective proliferation of ammoniaoxidizing bacteria (AOB) over nitrite oxidizing bacteria (NOB). Systemsand methods according to the disclosed subject matter include BNRreactor operating parameters that facilitate the proliferation of AOBover NOB and thus facilitate partial nitrification. To limit theoxidation of nitrite to nitrate and thus help AOB concentrations in aBNR reactor to build, low dissolved oxygen (DO) concentrations aremaintained during nitrification, which serves to prevent AOB bacteriafrom completing nitrification because AOB typically have a higheraffinity for oxygen than NOB. In addition, the BNR reactor is operatedwith an operational solids retention time (SRT) that facilitatesselective NOB washout. As the AOB begins to dominate over the NOB,higher free ammonia (FA) concentrations are found, which helps tofurther inhibit NOB growth.

Generally, the disclosed subject matter relates to systems and methodsfor controlling nitrification reactions so that partial nitrification isachieved. Referring now to FIG. 2, some embodiments of the disclosedsubject matter include a system 100 for achieving partial nitrificationin a biological nitrogen removal reactor 102. System 100 includes ameasurement module 103, a pH control module 104, a DO control module106, and an operational SRT control module 108.

Measurement module 103 includes testing apparatus for measuring pH,dissolved oxygen, and operational SRT in reactor 102. Measurements frommeasurement module 103 are relayed to pH control module 104, DO controlmodule 106, and operational SRT control module 108.

pH control module 104 includes apparatus such as a software program (notshown) residing on a computing device 110 for comparing pH measured inreactor 102 to a predetermined range for promoting growth of AOB. Insome embodiments, a substantial portion of the AOB in reactor 102 isrelated to Nitrosomonas europaea. pH control module 104 also includesapparatus such as a chemical storage tank 112 and a conduit 114 forproviding chemicals to reactor 102 to adjust pH in the reactor if it isnot within a predetermined range in an effort to bring it to within thepredetermined range. In some embodiments, the predetermined range for pHis from about 7.4 to 7.6. In some embodiments, chemicals for adjustingpH include sodium bicarbonate. Of course, other chemicals that will notinterfere with the growth of AOB can also be used for adjusting pH inreactor 102.

DO control module 106 includes apparatus such as a software program (notshown) residing on a computing device 116 for comparing DO measured inreactor 102 to a predetermined range for limiting the AOB fromcompleting nitrification. DO control module 106 also includes apparatussuch as an air supply 118, chemical supply 120, and conduits 122 foradjusting DO in reactor 102 if it is not within the predetermined rangeso that it is within the predetermined range. In some embodiments, thepredetermined range for DO is about 0.6 to 2.5 mg O2/L. In someembodiments, chemical supply 120 includes ammonium and DO is adjusted byinjecting a mixture of air and ammonium into reactor 102 via conduits122.

Operational SRT control module 108 includes apparatus such as a softwareprogram (not shown) residing on a computing device 124 for comparing theoperational SRT of reactor 102 to a predetermined range suitable formaintaining increasing concentrations of AOB in the reactor whilereducing concentrations of NOB in the reactor. Operational SRT controlmodule 108 includes can include alarms or alerts for adjustingoperational SRT in the reactor if it is not within the predeterminedrange so that it is within the predetermined range. In some embodiments,the predetermined range of the operational SRT is about 2.5 to 3.5 days.

System 100 is typically operated so that reactor 102 is at a temperaturesubstantially close to ambient. System 100 is configured to operateeither manually or automatically and in real time.

Referring now to FIG. 2, some embodiments include a method 200 ofcontrolling a nitrification reaction in a biological nitrogen removalreactor to favor partial nitrification of ammonia to nitrite instead ofcomplete oxidation of ammonia to nitrate. At 202, method 200 includesmaintaining a pH in the reactor within a range that promotes growth ofAOB. In some embodiments, the range of pH in the reactor is about 7.4 to7.6. At 204, a concentration of dissolved oxygen in the reactor ismaintained within a range that limits AOB from completing nitrification.In some embodiments, the range of dissolved oxygen in the reactor isabout 0.6 to 2.5 mg O2/L. At 206, an operational SRT is selected so asto be within a range suitable for maintaining increasing concentrationsof the AOB in the reactor while reducing concentrations of NOB in thereactor. In some embodiments, the operational SRT is about 2.5 to 3.5days. In some embodiments, the operational SRT is 3.0 days. At 208, aconcentration of free ammonia in the reactor increases over theoperational SRT thereby inhibiting growth of the NOB in the reactor. Insome embodiments, method 200 includes operating the reactor at atemperature substantially close to ambient. In some embodiments, method200 is practiced where a substantial portion of the AOB in the reactoris related to Nitrosomonas europaea.

Performance of systems and methods according to the disclosed subjectmatter was evaluated by creating, operating, and observing the operationof a laboratory-scale BNR reactor, which was operated according to thedisclosed subject matter. Observation of the laboratory-scale BNRreactor included DNA extraction, cloning, sequencing, and quantificationof AOB and NOB concentrations via real time qPCR. The cellconcentrations were converted to AOB and NOB biomass COD concentrations(X_(AOB) and X_(NOB)).

Referring now to FIG. 4, partial nitrification performance is high asreflected by long term stability of ammonia to nitrite oxidation.Initiation of steady-state was operationally defined as the first day ofoperation when an operational SRT of 3 days running average averagedammonia removal and nitrite accumulation that were both higher than 66%.Based on this criterion, steady-state based on bioreactor performancewas achieved within 47 days of continuous operation. After reachingsteady-state, long term stability of ammonia to nitrite oxidation wasreflected in 82.1±17.2% ammonia removal relative to influent ammoniaconcentrations. The major fraction of ammonia oxidized was to nitrite77.3±19.5% and not to nitrate 19.7±18.3%. Based on a nitrogen massbalance around the reactor, minimal nitrogen losses of 7±8.3% wereobserved. Bioreactor performance was transiently diminished by a nitriteshock load introduced to the reactor after 283 days.

Microbial population diversity and abundance in the bench-scale reactorwere also evaluated. Based on clone libraries constructed from DNAextracts obtained on two independent sampling dates (before and afterthe nitrite spike), most bioreactor AOB were closely related to N.europaea. Clones related to AOB such as Nitrosospira spp. or NOB,Nitrobacter spp. or Nitrospira spp. were not detected. Referring now toFIG. 5, the dominance of AOB determined via clone library analysis wascorroborated by routine qPCR results. As observed with sOUR measures,microbial abundance also varied quite dynamically, although bioreactorperformance was at steady-state for most of the study period. Duringsteady-state operation, AOB constituted 61±45% of the total bioreactorpopulation as COD. The corresponding steady-state Nitrobacter spp.related NOB fraction was much lower at 0.7±1.1%. As shown in FIG. 5, theincreased SRT, combined with the increased reactor nitriteconcentrations, following the nitrite spike resulted in a rapid andtransient increase in X_(NOB) concentrations. This trend suggested thatselect NOB populations remained viable and poised to proliferate in thepartial nitrification bioreactor, when the optimal conditions arose.

The population of AOB in the bench-scale BNR reactor operated accordingto systems and methods of the disclosed subject matter far exceeds thatof NOB, resulting in the nitrification process essentially stopping atnitrite without proceeding to nitrate. Thus, partial nitrification wasachieved.

Methods according to the disclosed subject matter provide advantages andbenefits over known methods. BNR strategies based on partialnitrification are more sustainable than those based on conventionalnitrification owing to their lower operating costs, e.g., 25% lessoxygen and 40% less electron donor for denitrification. Sustainedpartial nitrification was achieved by selective washout of NOB via acombination of free ammonia toxicity, low DO concentration, andoperation at an NOB limiting SRT. The imposed bioreactor operatingconditions enriched for distinct AOB (ecologically) and NOB(ecologically and biokinetically) populations compared to those inconventional activated sludge bioreactors.

Methods and systems according to the disclosed subject matter improvethe efficiency of nitrogen removal during waste treatment over knownsystems and methods. By performing partial nitrification, the costs inconventional treatment systems attributed to converting to and fromnitrate are avoided. This will result in significant savings for anypublic or private entity tasked with removing nitrogen from waste.

Another large source of nitrogen pollution is from animal agriculture.Systems and methods according to the disclosed subject matter could beapplied on a smaller scale to treat animal waste and reduce its toxicitybefore it is dumped into lakes and rivers. Since the input waste isessentially the same as a city's sewage, systems and methods accordingto the disclosed subject matter will apply equally well.

Systems and methods according to the disclosed subject matter can beused to treat various runoff waters containing ammonia. For example,landfill leachate is typically high in ammonia produced by thedecomposition of biological matter and requires treatment. Fertilizerrunoff from farms is also typically high in ammonia and requirestreatment. Systems and methods according to the disclosed subject mattercould also be used to remove nitrogen in lower concentrations frompolluted lakes and rives in areas with intensive agriculture.

Although the disclosed subject matter has been described and illustratedwith respect to embodiments thereof, it should be understood by thoseskilled in the art that features of the disclosed embodiments can becombined, rearranged, etc., to produce additional embodiments within thescope of the invention, and that various other changes, omissions, andadditions may be made therein and thereto, without parting from thespirit and scope of the present invention.

What is claimed is:
 1. A method of controlling a nitrification reactionin a biological nitrogen removal reactor to favor partial nitrificationof ammonia to nitrite instead of complete oxidation of ammonia tonitrate, said method comprising: promoting growth of ammonia oxidizingbacteria; limiting said ammonia oxidizing bacteria from completingnitrification; selecting an operational solids retention time within arange suitable for maintaining increasing concentrations of said ammoniaoxidizing bacteria in said reactor while reducing concentrations ofnitrite oxidizing bacteria in said reactor; and increasing aconcentration of free ammonia in said reactor thereby inhibiting growthof said nitrite oxidizing bacteria in said reactor.
 2. The methodaccording to claim 1, wherein said step of promoting includesmaintaining a pH in said reactor within a range that promotes the growthof ammonia oxidizing bacteria.
 3. The method according to claim 2,wherein said pH is about 7.4 to 7.6.
 4. The method according to claim 1,wherein said step of limiting said ammonia oxidizing bacteria fromcompleting nitrification includes maintaining a concentration ofdissolved oxygen in said reactor within a range that limits said ammoniaoxidizing bacteria from completing nitrification.
 5. The methodaccording to claim 3, wherein said dissolved oxygen in said reactor isabout 0.6 to 2.5 mg O2/L.
 6. The method according to claim 1, whereinsaid solids retention time is about 2.5 to 3.5 days.
 7. The methodaccording to claim 1, wherein said reactor is operated at a temperaturethat is substantially the ambient temperature.
 8. The method accordingto claim 1, wherein a portion of said ammonia oxidizing bacteria in saidreactor is Nitrosomonas europaea.
 9. A system for achieving partialnitrification in a biological nitrogen removal reactor, said systemcomprising: testing apparatus for measuring pH, dissolved oxygen, andsolids retention time in said reactor; a pH control module formaintaining pH in said reactor within a range that promotes growth ofammonia oxidizing bacteria; a dissolved oxygen control module formaintaining dissolved oxygen in said reactor within a range thatprevents ammonia oxidizing bacteria from completing nitrification; and asolids retention time control module for maintaining operational solidsretention time in said reactor within a range suitable for maintainingincreasing concentrations of said ammonia oxidizing bacteria in saidreactor while reducing concentrations of nitrite oxidizing bacteria insaid reactor; wherein said system increases a concentration of freeammonia in said reactor thereby inhibiting growth of said nitriteoxidizing bacteria in said reactor.
 10. The system according to claim 9,wherein said pH control module is effective to adjust the pH in saidreactor so that it is within a predetermined range.
 11. The systemaccording to claim 10, wherein said predetermined range for pH in saidreactor is about 7.4 to 7.6.
 12. The system according to claim 10,wherein said pH control module includes apparatus for adding sodiumbicarbonate to said reactor to adjust pH.
 13. The system according toclaim 9, wherein said dissolved oxygen control module is effective toadjust the dissolved oxygen in said reactor so that it is within apredetermined range.
 14. The system according to claim 13, wherein saidpredetermined range for dissolved oxygen in said reactor is about 0.6 to2.5 mg O2/L.
 15. The system according to claim 13, wherein saiddissolved oxygen module includes apparatus for injecting a mixture ofair and ammonium into said reactor to adjust dissolved oxygen.
 16. Thesystem according to claim 9, wherein said operational solids retentiontime control module is effective to adjust the operational solidsretention time in said reactor so that it is within a predeterminedrange.
 17. The system according to claim 16, wherein said predeterminedrange for solids retention time is about 2.5 to 3.5 days.
 18. The systemaccording to claim 9, wherein said reactor is operated at a temperaturethat is substantially the ambient temperature.
 19. The system accordingto claim 9, wherein said system is configured to operate automaticallyand in real time.
 20. The system according to claim 9, wherein a portionof said ammonia oxidizing bacteria in said reactor is Nitrosomonaseuropaea.